The blaze that closed a seven-mile stretch of the M1 was started deliberately, investigators said, as the Metropolitan police opened a criminal investigation into the incident.

The fire under an elevated section of the motorway in north London last Friday caused days of disruption as engineers battled to prevent a complete collapse of the severely damaged structure. Propped up with 200 tonnes of steel, the bridge reopened in both directions on Monday, but traffic will be limited by speed restrictions to prevent further damage to the reinforced concrete.

London fire brigade investigators who picked through the wreckage concluded the fire in a scrapyard in Mill Hill had been started deliberately and handed over the case.

The Met has appealed for information from anyone who was driving along the affected section of the M1 between 4am and 5am last Friday, when it is believed the arsonists struck. "The fire caused substantial damage and disruption and is being treated as suspicious," said a spokesman.

The blaze led to severe delays for football fans and marathon runners travelling to and from the capital at the weekend, as southbound traffic was barred between junctions 1 and 4 of the M1.

The Highways Agency hopes to expand the number of lanes open each way to three before Easter weekend. "The next step is trying to get three narrow lanes in each direction and we are hoping to get that done before Easter," it said.

Ministers have advised motorists to use alternative routes, such as the M40 or A1, because of the narrow lanes and speed restrictions on the M1.

The transport secretary, Philip Hammond, has asked the Highways Agency and Network Rail to assess the scale of the threat posed by businesses located near main rail routes and motorways.